State of the art logarithmic linear amplifiers are based on a circuit structure as illustrated in FIG. 1. Such a structure is disclosed in U.S. Patent Application No. 2002/0048109. The gain of these amplifiers commonly use two stages for increasing the gain range, which is given by the following equation:
                    Gain        =                                            1              +              x                                      1              -              x                                                          (        1        )            
The linear logarithmic gain (dB gain) variation range of the function is relatively restricted. Generally, the value of x of the gain expression for remaining substantially in the linear dB gain variation range is limited to −0.7<x<0.7.
The variation of the bias current in a transistor notably alters its transconductance, thus obtaining the desired behavior of the circuit according to equation 1. However, the variation of the bias current also affects the bandwith and the linearity of the amplifier.
The group delay of the amplifier circuit is inversely proportional to the bandwith. It would be very desirable to render the group delay independent from the gain to prevent interaction between the gain loop and the phase loop.
Variable gain amplifiers work with a fixed output voltage swing (fixed by the control loop) and the gain is inversely proportional to the input voltage swing. While it would be highly desirable that linearity should be inversely proportional to the gain, unfortunately the known amplifier structures such as the one depicted in FIG. 1 has the opposite behavior.